Extendible pre-stressed tubular structures such as those disclosed in U.S. Pat. Nos. 3,503,164 and 3,696,568 have been known for some time and their usefulness has been generally recognized. These pre-stressed structures or elements have found considerable use in space since they are capable of being rolled into a compact package and yet can be extended to form comparatively long structures which are light in weight compared to their length and volume. The comparative lack of gravitational forces in space also has contributed to the usefulness and effectiveness of such pre-stressed devices, since they were not exposed to such forces which might cause their distortion or damage. The possibility of damage or distortion due to gravitational forces may account for the comparative lack of usage of pre-stressed structures for non-space applications. Pre-stressed elements also form comparatively frail looking structures and this may also explain why they have found such limited usage.
There are however, a number of non-space applications where pre-stressed elements would offer various advantages. For instance, such structures can be used in place of many cables which are used in connection with various types of machinery. Since a cable can only normally be pulled and not pushed whereas a pre-stressed structure can both be pushed and pulled, a pre-stressed structure has various inherent advantages which a cable does not possess, such as the possibility of better control.
It has now been found that by suitably pre-stressing an element so that it has an appropriate cross-section having partially circular sections with appropriate radii, an appropriate structure can be formed that is useful lifting loads and yet at the same time is capable of being readily rolled into a compact configuration. Pre-stressed elements which have a cross-section that includes at least a portion of the circle are by no means new. An example of such a structure is disclosed in U.S. Pat. No. 3,434,254 which discloses a device that was particularly designed for use in space. The device illustrated in FIG. 1 of U.S. Pat. No. 3,434,254 has a central cross-section portion with a radius R and two exterior portions each having a radius R'. It should also be noted that the center of the circle with the radius R is offset by a distance h and that the centers of the circles for the central radius R and for the two exterior radii R' do not lie on one straight line. Furthermore, two such structures are used in tandem and they face each other and have their edges connected together. While the use of two such devices in tandem might have been satisfactory for space applications only one of such structures would appear to have limited usefulness in view of its limited strength.
It has also been recognized in the past that the edges of such extendible elements can cause damage by cutting or snagging and attempts have been made to avoid or prevent such damage. One such solution to this problem is presented in U.S. Pat. No. 3,696,568 which discloses a pre-stressed element whose exterior edges curl inward so that there are no exposed edges in the curled element. However, in most cases it is highly desirable not to have to design the overall configuration of the extendible element to reduce possible damage by its sharp edges.
Usually such pre-stressed elements are formed by a comparatively complex method such as that disclosed in U.S. Pat. No. 3,465,567 which involves heating the element and it would be desirable to simplify such methods.
The present invention overcomes many of the disadvantages present in the prior art devices and provides an extendible pre-stressed element with a unique cross-section which is strong in comparison to its weight which has provision for protection from its sharp edges. The method of the invention also permits the pre-stressed element with a unique cross-section to be formed easily without heating the element.